In general, histograms are created in a signal processing circuit for television signals to extract the characteristics of video signals. Applied technology for creating histograms has been introduced in the past. One example of a device employing such technology is disclosed in Japanese Laid-open Patent No. S58-63989.
FIG. 17 shows an example of a histogram which divides sampled input signals into more than one group based on the size of sample value. In FIG. 17, sampled signals are divided into four groups. In order to group the sample values, thresholds a1, a2, and a3 are set, and four intervals, S1, S2, S3 and S4 are then formed in accordance with these thresholds. Continuous input video signals are sampled and sample values allocated to one of the intervals S1, S2, S3, and S4. A histogram is created based on the intervals S1, S2, S3, and S4, and the counted number of samples (frequency) allocated to each intervals.
In a histogram generator such as the one of the aforementioned patents, however, the created histogram may be unstable. A sample value of the input signal that lies in small neighborhood of the threshold al may be judged by the histogram generator as belonging to the interval S1 at one moment while judged as belonging to the interval S2 at another moment, due to factors like noise.